Theoretical Study of Alkanedithiolated Gold Clusters
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Theoretical Study of Alkanedithiolated Gold Clusters J. M. Cabrera-Trujillo1 and R. Jiménez-Cataño2 1 Facultad de Ciencias, and 2Facultad de Ciencias Químicas, Universidad Autónoma de San Luis Potosí, 78000 San Luis Potosí, S.L.P. Mexico. E-mail: [email protected]; [email protected] ABSTRACT A reactive molecular dynamics simulation study on the structure, energetics, and chemistry of alkanethiolated gold cluster is presented. Through very recent reactive molecular dynamics force-fields developed by Järvi et al. [1], chemical reactions of alkanedithiolates and star-like shape gold nanoparticles are studied throughout octanedithiolates and stellated cuboctahedral gold clusters models [2] at room temperature. Structure, energetics, reactants, and some products of the reactions are preliminarily analyzed up to 25 ps. In general, preliminary results of this work are in agreement with those reported in the review by Love et al. [3]. INTRODUCTION Saturated hydrocarbon chains ended in thiol groups have been widely used to stabilize gold nanoparticles [3]. The well-known affinity between gold and sulfur allows covering a surface of gold atoms by hydrocarbon chains inert to water, air, and many substrates [4]. Moreover, thiols with hydrocarbon chains from 1 to 12 carbon atoms are easily found in the market with more than 97% purity. Longer chains are also easily found under request [5]. To allow strong bonds between gold and sulfur, thiols are converted to thiolates eliminating the hydrogen atom bonded to sulfur. This can be a spontaneous process on the gold surface [4] or accomplished by a reducing agent like sodium tetrahydridoborate, NaBH4 [6]. In the Templeton et al. [6] procedure, gold nanoparticles protected by a monolayer of thiolates are produced in a toluene solution containing the thiol, tetracloroaurate anions, AuCl4- (from HAuCl4), and NaBH4. Alkane dithiols can also be used instead of alkane thiols. The more common dithiols have a thiol group on each end of a methylene chain. Some of them are also commercially available, like 1,6-hexanedithiol, 1,8-octanedithiol, etc. These compounds have two ends capable to bond Au atoms, once converted to the thiolate form, or to bond a sulfur atom of another thiolate, producing longer dithiolate chains through a S-S link. The two ends could be bonded to Au atoms located in a single gold nanoparticle [7] or, more interesting, to Au atoms belonging to different gold nanoparticles. This situation may produce polymer-like structures, containing a web of alternating dithiol chains and gold nanoparticles which could have interesting properties [8]. On the other hand, there is special interest in gold nanoparticles with star-like shapes due to potential applications in medicine [9]. Recently, stellated nanoparticles have been synthesized and characterized by TEM and geometrical models to study their structure and energetic stability have been proposed [2,9]. From these models, the stellated cuboctahedral (SC) clusters have a rich surface: its morphology contains terraces with (111)
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